Spong in U.S. Pat. No. 4,097,895 entitled Multilayer Optical Record filed concurrently herewith and incorporated herein by reference, has described an ablative recording system whereby a focussed modulated light beam, such as a laser beam, is directed at an ablative recording medium. The recording medium comprises a light reflecting material coated with a light absorbing material on a substrate. The thickness of the light absorbing layer is chosen so that a maximum of light energy impinging on the organic layer is retained therein and is converted to thermal energy. This thermal energy causes the light absorbing material in the area struck by the light to sublime or ablate, thereby exposing selective portions of the reflecting layer. During readout, the contrast between the light absorbing layer and the reflecting layer is detected.
In a preferred embodiment of this recording system, a substrate which is flat, smooth and is a non-conductor of heat, is coated with a thin layer of a light reflecting material such as aluminum. The aluminum layer is in turn coated with a layer of an organic light absorbing material such as fluorescein.
Aluminum is an excellent light reflecting material for use in the above-described recording medium, since it can be readily applied to many substrates as a thin, non-grainy, highly reflective and adherent film. Further, aluminum can be evaporated at fairly low temperatures in a vacuum chamber. However, aluminum is also readily oxidized in the presence of oxygen to aluminum oxide (Al.sub.2 O.sub.3) and a thin monolayer, about 3-5 angstroms thick, forms on the surface of aluminum, even in a vacuum chamber, due to the presence of even trace amounts of oxygen. This thin layer of oxide is not necessarily detrimental to an aluminum light reflective material for use herein as a recording medium, since the reflectivity of the surface is still high and the oxidized layer is so thin that it does not noticeably affect the smoothness or optical quality of the surface.
However, it has been noted that the surface of the above light reflecting material changes with time, particularly when coated with a hydrophilic material such as fluorescein. This is caused by further oxidation of the aluminum due to diffusion of oxygen and water vapor from the fluorescein layer. This continued slow oxidation results in a degradation of the optical finish and a change in bonding strength of the light absorbing layer to the aluminum. This is a problem exacerbated after recording, when part of the aluminum is exposed to the atmosphere and part is covered with a light absorbing layer, resulting in different rates of continued oxidation.
Another problem has been that since aluminum itself is readily ablatable by a laser beam, if the power level of the beam is too high during recording, part of the aluminum is ablated along with the organic layer. The pitting in the aluminum degrades the signal to noise ratio obtainable during readout. It would be desirable to passivate the aluminum.